Increasing the circuit density on silicon wafers requires increasing cleanliness and purity on the device side of the wafer (device active region). Increased purity is obtained by forming collection sites (gettering sites) outside the device active region. The process of forming the collection sites is called gettering.
Gettering is divided into two types, internal gettering and external gettering. Internal gettering is achieved by forming gettering sites in the bulk of the wafer, while external gettering is characterized by forming gettering sites at the back surface of the wafer. External gettering can be divided based upon the methodology of gettering site formation for example sandblasting, polysilicon film deposition, laser damage, ion implantation and phosphorus diffusion. One side effect of using external gettering can be increased wafer warpage caused by the stress induced by the polysilicon film (residual stress).
Wafer warpage causes improper focusing during lithography. To increase purity on the device side of the wafer a gettering layer seeks to obtain a high crystal defect density (gettering sites), which unfortunately increase residual stress and promotes wafer warpage. When the front surface is mirror polished and the polysilicon film is left on the back surface only, the front warps in a concave (convex) form for a compressive (tensile) stress induced by the gettering layer.
Accordingly, it is desirable to have a method of external gettering that reduces the effect of warpage on a wafer and/or controls the warpage of a wafer.